Radiotherapeutic apparatus

ABSTRACT

The present invention seeks to provide a radiotherapeutic apparatus that mitigates the various problems found in the techniques such as tomotherapy, IMAT, IMRT and the like. It provides a radiotherapeutic apparatus comprising a source of radiation whose output is collimated by a multi-leaf collimator, and a patient support, the source being rotatable around the support and the support being translatable along the axis of rotation, thereby to move the source helically relative to a patient on the support. The leaves of the MLC are preferably oriented orthogonal to the axis of rotation, to simplify computation of the dose distribution. The apparatus thus moves the patient on the patient support system along the axis of rotation, in the longitudinal direction. Thus, the device has an effectively unlimited treatable volume in the longitudinal direction and avoids the limitations of IMAT and IMRT techniques whilst enabling the use of thin MLC leaves to give a high longitudinal resolution. The apparatus is preferably combined with an optimization system providing a computational service similar to that provided for IMAT and IMRT devices. Essentially the same computational techniques could be used, with appropriate changes to the input conditions and characteristic equations. The long aperture length (compared to tomotherapy) makes the radiation delivery efficient and therefore the delivery of high doses a practicality; hypofractionation and radiosurgery therefore become possible over large treatable volumes.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/GB2006/000725, filed 1 Mar. 2006,which claims priority to Great Britain Patent Application No. 0504897.0filed on 10 Mar. 2005 in Great Britain. The contents of theaforementioned applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a radiotherapeutic apparatus.

BACKGROUND ART

It is known that exposure of human or animal tissue to ionisingradiation will kill the cells thus exposed. This finds application inthe treatment of pathological cells. In order to treat tumours deepwithin the body of the patient, the radiation must however penetrate thehealthy tissue in order to irradiate and destroy the pathological cells.In conventional radiation therapy, large volumes of healthy tissue canthus be exposed to harmful doses of radiation, resulting in prolongedrecovery periods for the patient. It is, therefore, desirable to designa device for treating a patient with ionising radiation and treatmentprotocols so as to expose the pathological tissue to a dose of radiationthat will result in the death of these cells, whilst keeping theexposure of healthy tissue to a minimum.

Several methods have previously been employed to achieve the desiredpathological cell-destroying exposure whilst keeping the exposure ofhealthy cells to a minimum. Many methods work by directing radiation ata tumour from a number of directions, either simultaneously frommultiple sources or multiple exposures from a single source. Theintensity of radiation emanating from each source is therefore less thanwould be required to destroy cells, but where the radiation beams fromthe multiple sources converge, the total intensity of radiation issufficient to deliver a therapeutic dose.

The treatment may be spread over a number of days or weeks, to allowrecovery of the healthy tissue. This should recover more quickly, as itreceived a lesser dose. Accordingly, repeated doses spaced over timewill eventually take a greater toll on the pathological cells.

It is therefore important to deliver the radiation as accurately aspossible to the pathological cells whilst minimising the dose to healthytissue. Progress in this regard allows greater individual doses to bedelivered in each treatment step, thereby reducing the total number oftreatments required and, in fact, reducing the overall dose delivered toa patient. For example, a prescription of 35 instances of a 2 Gy dosemight be replaced by 15 instances of a 3 Gy dose, a technique known as“hypofractionation”. Taken to its logical extreme, this might bereplaced with a single 45 Gy dose if the dosage delivered to healthytissue can be reduced significantly. This approach, referred to moretypically as radiosurgery, will evidently offer advantages to thepatient in that fewer treatments are required and there is no risk ofinconsistent positioning between treatments.

Various methods have been proposed to reduce the dose the healthy tissuewhilst maximising the dose to pathological tissue. The simplest is todirect the beam of radiation from a number of directions. Thus, at theco-incidence of the beams the dosage with be approximately ‘n’ times thedosage delivered to other areas, where ‘n’ is the number of directionsemployed.

Collimation can also be employed to limit the beam size to the minimumrequired to illuminate the pathological tissue. Multi-leaf collimators(MLCs) are known, such as that described in EP-A-0,314,214, and theseare able to shape the beam to a desired outline.

In WO-A-02/069,349, we proposed a system whereby a beam of radiation wasswept across the region of interest whilst its width was modulated. Thisoffers the great advantage of an unlimited length to the treatment area.

In “rotational conformal” collimation, a radiation source is rotatedaround the patient and collimated with an MLC. The shape of the MLCcollimation is varied with the angle of approach so that the width ofthe beam always conforms to the projected outline of the tumour as seenin the beam direction. This is useful for some shapes but deals poorlywith concavities or re-entrant shapes.

IMAT techniques are described in U.S. Pat. No. 5,818,902. This developsthe rotational conformal technique further by allowing repeatedrotations around the patient. In this way, doses can be built up in thetumour area step-by step. To decide on the MLC shapes and directions,computational methods are used. Each voxel of the region of interest isassigned a “cost function”, which reflects the cost associated with aspecific dose. Thus, for example, a voxel in the tumour area has a highcost associated with a low dose, whereas a voxel in a healthy area willbe opposite. Some sensitive areas such as the spine and the digestivetract can be given cost functions that place a particularly high cost ondoses above a certain critical level. Computational processes then seekto minimise the cost function by manipulating the delivery options. IMATcan provide exceptional dose distributions.

IMRT is similar in its computational principles to IMAT, but providesfor a series of MLC-shaped beams from the same direction. Thus, thecomputational load is somewhat reduced.

Tomotherapy is a treatment technique described (for example) in‘Planning Evaluation for complex lung cancer cases using HelicalTomotherapy’ T. Kron et al. Phys. Med. Biol. 49 (2004) 3675-3690. Inthis technique, a modulatable fan beam is produced from a source that isrotated around the patient in a helical fashion. The beam's intensitycan be modulated by elements that slide into and out of the path of thefan beam across its width. The dose can be very conformal and the dosedistributions achieved are impressive.

SUMMARY OF THE INVENTION

There are distinct problems or limitations with all of the abovetechniques.

The system of WO-A-02/069,349, for example, only employs a singleapproach direction. IMAT and IMRT offer excellent dose distributions, asnoted, but pressure towards greater resolution requires a higherresolution MLC, which tends to have a smaller aperture. Thus, thetreatable volume becomes limited.

Helical tomotherapy offers a delivery efficiency that is exceptionallylow. In order to achieve comparable plans, a 25 mm Fan Beam Thickness(FBT) is used, together with a Modulation factor (MF) of at least3—lower values (higher efficiencies) lead to unacceptable plan quality.This results in 2 Gy dose delivery taking approximately 20 minutes beamon time. The dose rate of the beam source is approximately 10 Gy/mini.e. in 20 minutes the machine is capable of delivering 200 Gy. Thismeans that the efficiency of dose delivery is about 1%. Typically usinga conventional MLC, IMRT techniques offer an efficiency of about 500%.This means that to achieve the same leakage dose to the patient, theshielding of a tomotherapy machines needs to be 50 times as good. Italso means that the machine consumes about 50 times as much electricalpower to deliver each fraction, and generates 50 times as much heat.This may also limit the lifetime of other components.

This also means that hypofractionation techniques are not practicable inthe context of helical tomotherapy. A 20 Gy fractional dose would take200 minutes to deliver i.e. 3½ hours beam on time. Typically, 20 minutesis regarded as the maximum length of time that a patient can lie still.Radiosurgery is less practicable still, as a 50 Gy dose would take 500minutes, i.e. 8½ hours beam on time.

The present invention seeks to provide a radiotherapeutic apparatus thatmitigates the various problems found in the above techniques. Ittherefore provides a radiotherapeutic apparatus comprising a source ofradiation whose output is collimated by a multi-leaf collimator, and apatient support, the source being rotatable around the support and thesupport being translatable along the axis of rotation, thereby to movethe source helically relative to a patient on the support.

It is preferred that the leaves of the MLC are oriented orthogonal tothe axis of rotation, as this will considerably simplify computation ofthe dose distribution. In such a case, it is envisaged that the MLCwould not be capable of rotation, distinct from a conventional treatmentmachine.

The apparatus thus moves the patient on the patient support system alongthe axis of rotation, in the longitudinal direction. Thus, the devicehas an effectively unlimited treatable volume in the longitudinaldirection and avoids the limitations of IMAT and IMRT techniques.Despite this, thin MLC leaves can still be used to give a highlongitudinal resolution.

A limited number of MLC leaves can be used, as the longitudinal motionextends the treatable length. This simplifies the engineering comparedto a conventional MLC, where the number of leaves is required to be highenough to cover the treatable area. In this way, the use of thinner MLCleaves for higher resolution no longer implies a smaller treatmentfield.

The apparatus is preferably combined with an optimisation systemproviding a computational service similar to that provided for IMAT andIMRT devices. Essentially the same computational techniques could beused, with appropriate changes to the input conditions andcharacteristic equations.

Such a device offers a large number of treatment variables that can beoptimised, thereby enabling similar or better dose distribution qualityas compared to IMRT and IMAT. However, the long aperture length(compared to tomotherapy) makes the radiation delivery efficient andtherefore the delivery of high doses a practicality; hypofractionationand radiosurgery therefore become possible over the large treatablevolume that was previously only available via tomotherapy.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 is a schematic illustration of the geometrical arrangementemployed in the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows, in outline, the apparatus used by the present invention. Apatient support 10 is provided that is moveable along a longitudinalaxis 12. A source of radiation 14 is also provided, on a mount (notshown) that allows it to be rotated around the longitudinal axis 12.Thus, the longitudinal axis of the patient support and the rotationalaxis of the radiation source are aligned. The source is directed towardsthe axis 12, such that its beam 16 intersects with the axis.

A multi-leaf collimator (MLC) 18 is provided for the radiation source14. The leaves 20 of the MLC are oriented perpendicular to the beam 16and to the axis 12. Accordingly, the leaves are in line with thetangential direction of the source 14 along its rotary path. Thistherefore collimates the radiation beam from a cone to what can beconsidered as a plurality of fan beams whose included angle iscontinuously variable. A computational means 24 is also provided forcalculating leaf positions for the multi-leaf collimator 18 as afunction of rotation angle of the source 14 to achieve a specified dosedistribution.

As the source 14 (and associated collimator 18) is rotated around theaxis 12, the patient support 10 is moved along the axis. Thus, relativeto a patient on the support 10, the source describes a helix 22.

This idea is probably easiest to understand by considering a specialcase, in which the patient is moved by the width of one MLC leaf forevery rotation of the treatment machine. However, it should beunderstood that the invention is applicable in other cases. In thisspecial case the radial extent of the treatment field in any transaxialplane is defined by successive leaves in the MLC for successiverotations of the machine. Thereby the dose in a given transaxial sliceis built up by the size of the apertures defined by successive leafpairs. This is analogous to the multiple rotations of the IMAT techniqueas described in U.S. Pat. No. 5,818,902.

Increasing the FBT of a tomotherapy device in order to reduce the “beamon” time and improve efficiency merely degrades the dose distribution inthe sup/inf direction to an unacceptable degree. Decreasing the FBT toimprove the dose distribution in the sup/inf direction will increase thebeam on time commensurately i.e. 10 mm FBT will result in a beam on timeof 50 minutes—an efficiency of dose delivery of about 0.4% or 1 part in250. According to the present invention, the width of the leaves wouldideally be about 3 mm at the isocentre and there should be at least 40leaves in each opposing bank. A greater or lesser number of leaves couldbe provided, such as 30, 20, 10 or even at least 3. A total of 40 suchleaves would make the length of the collimator 120 mm.

The efficiency is likely to scale by the length of the collimator. Thus,techniques similar to tomotherapy are likely to be 5 times moreefficient (120 mm vs. 25 mm). Also, our experience of IMAT techniquesshows that the efficiency is typically quite high, of the order of 30%.

The machine could be rotated continuously, but the radiation could beturned off during portions of the arc that do not require radiation.This 30% is of course degraded by the ratio of the tumour length to thecollimator length—a 240 mm tumour length would degrade the efficiency to150%. So these arguments conclude that the efficiency of such atechnique is likely to be between 5 and 15%. This is worse than aconventional MLC technique but much better than the tomotherapytechnique.

Using leaves of width 3 mm will give a substantially better quality ofdose distribution than the FBT of 25 mm of the Tomotherapy techniquedescribed. This is because the geometric penumbra of the distributionsin the longitudinal direction is likely to be 3 mm and 25 mmrespectively for these two techniques.

If the tumour length is less than the length of the collimator the samemachine can be used to treat the tumour using a conventional IMATtechnique, by leaving the patient support system stationary. The machinecan thus be made to deliver both these techniques without modification.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention.

1. A radiotherapeutic apparatus comprising a source of radiation whoseoutput is collimated by a multi-leaf collimator, and a patient support,the source being rotatable around the support and the support beingtranslatable along the axis of rotation, thereby to move the sourcehelically relative to a patient on the support, wherein the multi-leafcollimator comprises at least three pairs of opposing leaves, each leafmovable such that the radiation output has an included beam angle thatis continuously variable to spatially modulate the beam, wherein themulti-leaf collimator is rotationally fixed relative to the source ofradiation.
 2. A radiotherapeutic apparatus according to claim 1, furthercomprising computational means for calculating leaf positions for themulti-leaf collimator as a function of rotation angle of the source toachieve a specified dose distribution.
 3. A radiotherapeutic apparatusaccording to claim 1 wherein the multi-leaf collimator has at least 10leaves.
 4. A radiotherapeutic apparatus according to claim 1, whereinthe multi-leaf collimator is oriented such that the leaves of thecollimator are movable in a direction orthogonal to the axis ofrotation.
 5. A radiotherapeutic apparatus according to claim 1, whereinthe multi-leaf collimator has a length at the isocentre in a directionparallel to the axis of rotation of between 60 mm and 120 mm.
 6. Aradiotherapeutic apparatus comprising a source of radiation whose outputis collimated by a multi-leaf collimator, and a patient support, thesource being rotatable around the support and the support beingtranslatable along the axis of rotation, thereby to move the sourcehelically relative to a patient on the support, wherein the multi-leafcollimator is oriented such that the leaves of the collimator aremoveable in a direction orthogonal to the axis of rotation to spatiallymodulate the beam, and wherein a pitch of said helical motion is equalto a width of a leaf of the multi-leaf collimator.
 7. A radiotherapeuticapparatus comprising a source of radiation whose output is collimated bya multi-leaf collimator, and a patient support, the source beingrotatable around the support and the support being translatable alongthe axis of rotation, thereby to move the source helically relative to apatient on the support, wherein the multi-leaf collimator comprises atleast 3 pairs of opposing leaves, each leaf movable such that theradiation output has an included beam angle that is continuouslyvariable to spatially modulate the beam, and wherein the multi-leafcollimator has a length at an isocentre in a direction parallel to theaxis of rotation of between 60 mm and 120 mm.
 8. A method of treating apatient using a radiotherapeutic apparatus, the radiotherapeuticapparatus comprising a source of radiation whose output is collimated bya multi-leaf collimator, and a patient support, the source beingrotatable around the support and the support being translatable alongthe axis of rotation, the method comprising: simultaneously rotating thesource around the support and translating the support along the axis ofrotation, thereby moving the source helically relative to a patient onthe support; and adjusting each of the leaves of the multi-leafcollimator such that the radiation output has an included beam anglethat is continuously variable to spatially modulate the beam.